Example 1: HN needs JS only for voting so I don't enable it here unless I want to vote a post or a comment. However HN's JS is trivial and can be easily inspected so I might want to enable it permanently as well.
Example 2: YouTube requires JS from 6 domains but I can keep four of them disabled and still be able to view videos. I really don't need googlesyndication.com, plus.googleapis.com and googletagservices.com (among the other things I'm missing comments but who cares). Some of them are probably just spyware. I also don't need google.com but Google Maps don't work without it so I probably already enabled it before going to YouTube.
Example 3: TechCrunch is a nightmare of nested JS requires. However I can read it without enabling any JS and it's also much faster to load.
If your site is (example) an Angular single page application I'll probably enable enough JS to make it move and check it out. If there are so many JS dependencies that I can't find the right combination and I see JS coming from well known spyware domains I might decide to skip that site completely. There are sites like that. If I'm really interested I fire up Chrome and give them a try in there. Actually, I might install the new Opera only for using it as a honeypot for that kind of sites.